This invention relates to nuclear reactor core sub-assemblies and, in particular, to the neutron shielding sections which may be incorporated in the upper ends of such sub-assemblies. The neutron shielding section may comprise an array of spaced, generally parallel, axially extending tubular pins containing pellets of a suitable neutron absorbing material such as boron carbide. Over the life of a typical fuel sub-assembly, it is found the neutron absorbing capacity of the neutron shielding section is only partly depeleted, primarly over a short axial section nearest to the fuel-bearing section of the sub-assembly.